FIG. 22 illustrates exemplary principal components of a conventional image forming device 1 in which electrophotography is used. An electrophotographic image forming device includes a photoconductor serving as an image carrier. An example of the photoconductor is a photoconductive drum 10 formed into a drum shape. Upon start of an image forming operation, the photoconductive drum 10 is rotated in a direction indicated by an arrow. Then, a surface of the photoconductive drum 10 is sequentially opposed to a primary charging unit 11, an exposure unit 15, a developing unit 12, a transfer roller (transfer unit) 13 and a cleaner 14. Specifically, first, the primary charging unit 11 electrically charges the surface of the photoconductive drum 10 uniformly. The exposure unit 15 emits an exposure light L that is a laser light. The exposure light L removes electric charges from the surface of the photoconductive drum 10 in accordance with image information. As a result, an electrostatic latent image is formed on the surface of the photoconductive drum 10. This electrostatic latent image is visualized by a developer (including toner) of the developing unit 12, and a toner image is formed on the photoconductive drum 10.
On the other hand, a recording paper P contained in a recording paper feed cassette 17 is conveyed in synchronization with formation of a toner image on the photoconductive drum 10. When the recording paper P passes through between the photoconductive drum 10 and the transfer roller 13, the toner image on the photoconductive drum 10 is transferred onto the recording paper P. Thereafter, the recording paper P having an unfused toner image thereon through transfer is guided by a guide plate 23, and conveyed to the inside of a fuser 20.
The fuser 20 includes: a heating rotator internally having a heater; and a pressurizing rotator provided so as to be rotated while being abutted against the heating rotator. The heating rotator is, for example, a heating roller 21 internally including a roller-like heater. The pressurizing rotator is, for example, a pressurizing roller 22 covered with an elastic body such as rubber. The rollers 21 and 22 are pressed to each other in an opposed manner. A pressed region between the rollers 21 and 22 serves as a fusing nip portion N. When the recording paper P passes through the fusing nip portion N, the recording paper P is heated and pressurized. As a result, the unfused toner image is fused onto the recording paper P, and becomes a permanent image.
The recording paper P on which image formation has been performed is then discharged to the outside of the image forming device. Furthermore, transfer residual toner or the like, remaining on the photoconductive drum 10 after the end of transfer, is removed by the cleaner 14. The image forming device is capable of repeatedly carrying out image formation in this manner.
Detection of temperature of the fusing nip portion N includes a contact type method and a non-contact type method. Conventionally, in the contact type temperature detection, a contact type temperature detector has been brought into contact with the heating roller 21 in order to detect a surface temperature of the heating roller 21. Then, based on the temperature detected by the contact type temperature detector, the temperature of the heating roller 21 has been controlled by a heater. Although not illustrated in this example, a thermistor has been used as the contact type temperature detector. The thermistor is brought into contact with the surface of the heating roller 21. Therefore, due to an abnormality such as a disconnection or a short circuit, and damage to the surface of the heating roller 21, abnormalities have frequently been caused in fused images. When the surface of the heating roller 21 is damaged, it is necessary to replace the rollers 21 and 22 or the entire fuser (cartridge). As a result, an increase in replacement cost will be caused. Moreover, the thermistor is normally attached to a fusing unit. Hence, when the fusing unit is replaced, the thermistor is also concurrently discarded. Accordingly, the replacement is not preferable not only in terms of cost but also in terms of resource saving.
Further, a device in which an induction heating type heater is used and a temperature control operation can be increased in speed is also put to practical use nowadays. However, in a system in which the surface temperature of the heating roller 21 is detected by using the conventional thermistor, there has been a disadvantage that a temperature detection response is slow, thus preventing a proper function as a device. In particular, in an induction heating type fuser in which energy conservation of a heater is taken into consideration, power consumption is required to be low during device standby, and a fuse temperature is required to rise quickly only during use. In other words, a detector having a high responsiveness is demanded.
Therefore, there has been proposed non-contact type temperature detection that is carried out using an infrared temperature sensor such as a thermopile. Examples of techniques in which non-contact type temperature detection is adopted include techniques of Patent Literatures 1, 2 and 3.